Beverage dispensing machines conventionally employ an ice water tank in which the evaporator coil of a refrigeration unit is placed as well as beverage tubing coils through which beverage product (syrup, carbonated water and water) flows. The temperature of the ice water tank is ideally maintained at 32.degree. F. to chill the water and syrup when dispensed through the machine's dispensing valve. Chilling of the beverage product occurs by conductive heat transfer across the tubing wall. To satisfy peak demand, the refrigeration unit is operated to build an ice bank about the evaporator coils so that the ice will provide an additional heat sink or cold storage to compensate for increased flow of the warmer fluids in the water and syrup coils. Chilling of the beverage product causes some of the ice to melt. The compressor of the refrigeration unit is then operated to replenish the ice.
The ice bank size must be controlled within a specified size range. For example, if the ice bank is too small, there may not be enough cold storage to satisfy periods of high cooling demand. However, if the ice bank becomes too large, it may grow into the beverage product coils causing the beverage product to freeze and rendering the beverage dispenser inoperable.
An ice bank control is conventionally used to cycle the refrigeration compressor and maintain the ice bank within an acceptable size range. Conventional ice bank controls use a sensor immersed at a preset location in the ice water tank to detect the presence of ice. As ice surrounds the sensor, the control detecting the presence of ice switches the compressor off. As the ice gradually melts away from the sensor, the control no longer detects ice and switches the compressor on. The cycle repeats itself indefinitely.
Two types of ice bank controls, mechanical and electronic, are in conventional use. The most popular are the mechanical controls which have been used for several decades. These controls typically employ a sensing bulb immersed in the ice water tank. The bulb is filled with water which itself freezes when surrounded by ice. When the bulb water freezes, the water (now ice) expands and pushes against a rubber diaphragm constructed in the sensing bulb. The diaphragm in turn pushes against a non-freezing ethylene glycol solution and pressure developed in the glycol solution is transmitted via a capillary tube to a piston assembly. The piston assembly, located outside the ice water tank, expands a rubber cup to push a piston against a spring lever mechanism which in turn actuates an electrical switch to deenergize the compressor. As the ice bank melts away from the sensing bulb, the reverse process occurs and the switch closes to actuate the compressor.
The mechanical control has been popular for many years because of its low cost and simplicity of operation. However, the control is very unreliable due to manufacturing variances and simply inherent mechanical wear. For example, faulty diaphragms or seals, leaking glycol, sticking pistons and improperly formed levers often cause intermittent compressor cut-in or cut-out. In a worst case failure mode, the mechanical control may cause the compressor to run continuously. This can cause the entire ice water tank to freeze up and extensively damage the beverage dispenser.
Electronic ice bank controls have been developed in recent years to provide increased reliability and this invention relates to an electronic control. Electronic control systems use an electrode assembly immersed in the ice water tank to sense the presence of ice. In its basic application, a low alternating current voltage (typically 9 volts) is applied to one pole of the electrode. Some electronic controls use pulsed direct current. Another electric pole is referenced to ground. Ice having a much higher electrical resistance than water, can be detected by comparison of electrical resistance across the electrode poles. A control board electrically connected to the electrode assembly is used to make the resistance comparisons and provide output switching action to operate the compressor.
U.S. Pat. Nos. 4,008,832 and 4,497,179 illustrate conventional electronic controls in which two probes are placed in the ice water tank in closely spaced alignment with the evaporator coil. The probe furthest from the evaporator senses water and the probe positioned closest to the evaporator senses ice. The compressor is cycled on when the ice probe detects water and off when the water probe detects ice. .While such arrangements as disclosed in the '832 and '179 patents have proven more reliable than the mechanical sensor arrangement described above, they are susceptible to failures in that contaminants, such as syrup within the ice water tank, can lower the freezing point of the tank. Water in the water coil then freezes rendering the dispenser inoperable. Still further, as a function of time, deposits from the ice water tank, resulting from evaporation for example, varies the resistivity of the probes adversely affecting their readings. In this connection, U.S. Pat. No. 4,823,556 teaches the use of four separate probes, two of which generate a resistance signal depending on the actual ice water tank conditions which then serves as the basis upon which ice and water probe signals are compared against to cycle the compressor off and on. The assumption is that all the probes will uniformly degrade so that the comparison will be viable. U.S. Pat. No. 5,022,233 offers another solution to the drift and/or probe degradation problem. In the '233 patent only one probe, precisely positioned where the desired ice-water interface is desired to occur, is used and the circuitry for shutting off and on the compressor includes a programmable microprocessor that compares the readings obtained over time and automatically correlates or adjusts them to the desired beta curve to account for drift.
In summary, a number of problems arise in conventional, electronic ice bank control systems which can be attributed to the fact that the sensor or the probe is in contact with the contents of the ice water tank. As noted, water deposits lead to contamination of the probe affecting its readings. Impurities such as syrup in the tank adversely affects the controls by lowering the freezing temperature of the tank. Because the sensor must be immersed in the tank an electrical short between the lead wires, caused by faulty insulation, can result. Stray voltage in the ice water tank can be transmitted to the electrodes. The prior art teaches to address the problems by using circuitry and/or software downstream of the probe in the control circuit. This approach increases the price of a system which is already more expensive than the mechanically equivalent system discussed above. Fundamentally though, the prior art reacts to the problem instead of addressing the problem. Should the control circuit be designed to not accurately respond to the problem encountered by the probe, or worse yet, fail to address the specific malfunction of the probe, the system will fail.
In addition to this inherent problem present in the electronic control systems of the prior art, special steps must be taken by means of specially designed bracket/spacers to accurately place the probe in desired spaced and orientation relationship to the evaporator coil. This necessitates disassembly or removal of the refrigeration deck of the beverage dispenser to gain access to the evaporator coils. The bracket has to be designed and applied in such a manner that the sensor doesn't move while ice grows and dissipates about it. When several sensors are used, typically encased in a bulb attached to the evaporator coil, care must be taken to assure that the sensors extend on a radial line from the center of the evaporator tubing. Such requirements make it difficult and/or expensive to retrofit mechanically equipped ice bank control beverage dispensers with electronic ice bank controls. It also makes replacing failed sensors difficult.